PROJECT SUMMARY The CREB-binding protein (CBP) and its paralog p300 are master transcriptional coactivators that integrate numerous signaling pathways and play critical roles in cell proliferation, differentiation, apoptosis, and DNA repair. The biological functions of CBP and p300 are largely exerted through multiple transcription factor interaction domains as well as a histone acetyl transferase (HAT) domain. However, a structural model of how transcription factor complexes with CBP and p300 modulate coactivator function is lacking. A well-studied cellular partner of CBP and p300 is the tumor suppressor protein p53. Several studies suggest that upon DNA damage CBP/p300 is recruited by p53 to modify chromatin and aid in transcriptional activation of p53 target genes. It has been hypothesized that each of the four transcriptional activation domains of an active p53 tetramer bind to four separate domains of a single CBP/p300 molecule, resulting in increased avidity that further stabilizes the p53-CBP/p300 complex and enhances p53-mediated transcription. However, the influence of p53 binding on local and global conformational changes in CBP/p300 and its effect on histone acetyltransferase activity has yet to be defined. Using a combination of single particle electron microscopy, solution biophysical and biochemical methods, the goal of the proposed work is to develop a comprehensive structure-function model of CBP/p300 in p53-mediated transcriptional activation.